Allow building official releases with MSVS 2017 without doing it from
its "developer command prompt" by using the vswhere utility to set up
the paths correctly.
Closes#18083.
VS2017 environment bat files change the working directory so the build
directory is returned to after calling them.
A warning has been added that the VS150COMNTOOLS environment variable
needs to be set or a VS2017 command prompt needs to be used for VS2017
builds. MS no longer sets this variable on install.
For vc110 and vc120 builds the x64 switch has been changed to x86_amd64.
Closes#18075.
Simplify the officialbuild.bat script by only checking for "vcXX" instead of
accepting both them and "vsYYYY".
Also add instructions for using it in the corresponding HOWTO.
Closes#17514.
Use a single script for all compiler versions, another simple script just
defining wxWidgets version and, finally, a script building all the binaries
for all the compilers.
Closes#17510.
